Process for improving the mechanical properties of magnesium and its alloys



Patented Aug. 22, 1933 UNITED STATES PROCESS FOR THE MECHAN- ICAL PROPERTIES OF MAGNESIUM AND ITS ALLOYS Walther Schmidt, Hans Bothmann,

and Jose! Ruhrmann, Bitterfeld, Germany, assignors, by

mesne assignments,

to Magnesium Development Corporation, a Corporation of Delaware No Drawing.

Serial No. 434,820, 1929 Application March 10,

and in Germany March 11,

6 Claims. (CL 148-21.3)

The present invention relates to a method of improving certain mechanical properties, particularly the yield strength in compression, of magnesium and magnesium alloys and of products made therefrom.

When magnesium and magnesium alloys are subjected to plastic deformation at elevated temperatures, we have observed that the improvement of the mechanical properties obtained by virtue of such deformation does not, as is the case with other metals, extend to all the mechanical properties, but is more or less limited to an improvement of the tensile strength and the properties connected therewith (such as yield strength in tension, elastic limit and elongation under tensile stresses), whereas the improvement of the elastic qualities with respect to stresses having the character of compression or of torsion is either considerably smaller or even practically absent. Thus, the yield strength in compression which in the untreated cast metals closely equals their yield strength in tension, after plastic deformation rarely exceeds about one half of the latter.

A further improvement of the mechanical properties of magnesium and magnesium alloys when plastically deformed at elevated temperatures can be obtained in a known manner by a subsequent deformation at ordinary temperatures. By this procedure the absolute values of the yield strength in tension as well as the yield strength in compression are simultaneously raised but their unfavorable ratio remains sub-' stantially unaltered.

Again, the yield strength in torsion which, as a rule, in the case of metals amounts to about one half of the yield strength in tension does not, in the case of magnesium and magnesium alloys, exceed about 25 percent of the yield strength in tension after plastic deformation has taken place under ordinary conditions and at elevated temperatures. Nor is this ratio perceptibly improved by a subsequent deformation and therefore it cannot, in this manner, be brought to the level prevailing in other metals.

The numerous technical applications of magnesium and its alloys render an improvement of this unfavourable relation between the yield strength in tension and the above mentioned mechanical properties, particularly the yield strength in compression, desirable. It is an object of the present invention to improve this relation. I

In the following general statement, the invention is described with respect to its application eign metals. The term to magnesium only. It should, however, be well understood that it is equally applicable to all alloys of magnesium in which this metal largely predominates, i. e. all alloys of magnesium containing not more than about 25 percent of formagnesium", when employed in the following, therefore includes these alloys as well unless the contrary is especially mentioned.

When subjecting magnesium to plastic deformation at elevated temperatures, preferably between 200 and 500 C., the rate at which deformation is efiected is, similar to other metals, for reasons of economy usually kept sufliciently high to be just compatible with the structural 7o coherence of the material under deformation. This rate of deformation, which may justly be termed the standard one, substantially depends on the temperature at which deformation is effected and on the individual plastic properties of the particular metal or alloy concerned. The range covered by the standard rate of deformation is, according to the foregoing, characterized by a speed of working which is, on the one hand, slightly below the excessive speed of deformation causing a fracturing of the metal, and on the other hand, will rarely, if ever, fall below two thirds of that speed.

Some specific examples will serve to illustrate the exact meaning of this statement.

The speed of working of a die extrusion press, once the temperature of working and the material to be extruded are given, is governed by the specific pressure brought to bear upon the material. When in a. particular instance it is observed that the application of a pressure exceeding, say, kg./sq. mm. results in the production of cracked or disrupted bars, whereas at a pressure of 65 kg./sq. mm. the bars are perfect and issue from the die at a speed of, say, 100 mms. length per second, it would seem unreasonable to reduce the pressure to, say, 30 kg./ sq. mm., this resulting in bars which are apparenthr none the more perfect and only leave the die at a speed of, say, 30 mms. per second. A 100 speed of the bars of about to 100 mms. would, in this case, thus correspond to the standard rate of deformation.

In the case of a rolling mill, the speed of deformation may be represented by the decrease 105 of the thickness of the billet effected by each pass, in percentages of the original thickness.

If a billet of a particular alloy, when worked at a certain temperature, is found to crack when no adjusting the rolls to a decrease of above 20 percent, while remaining uncracked when a decrease of 20 percent and below is selected, it would be devious to apply a decrease which is only a fraction of 20 percent, as long as there is no cogent reason for such an abnormal procedure. In this case, therefore, a decrease ranging between about 16 and 20 percent would be considered the standard rate of deformation.

Independently, the speed of deformation in a rolling mill may be represented by the speed at which the slabs are passed through the rolls, i. e. the speed of the rolls themselves. Also in this respect there always exists a standard rate of working which must not be exceeded materially if a breaking up of the metal is to be avoided, and which, on the other hand, will always be chosen as high as possible for reasons of economy.

In the first place, we have found that the numerical relation between the tensile properties on the one hand and the other elastic properties mentioned above on the other hand, in the case of magnesium, is greatly improved by carrying out plastic deformations at elevated temperaturesat a rate which falls distinctly below the standard range and amounts to less than half of the latter. We have further found that the same result is obtained when the metal while being plastically deformed at an arbitrary rate below the upper limit of cohesion but, including the standard range, is quenched coincidentally with the ceasing of the effect, upon the metal, of the external forces causing the deformation. The lowering of the temperature throughout the material by virtue of the chilling must be such as to efficiently prevent any further change in the structure of the constituting crystals with respect to their size and orientation. The quenching of the metal can be performed by means of cold or tepid water, oil, compressed air, or the like. In any case, however, it is essential that the cooling medium should be brought to bear upon the metal in immediate vicinity of the point of issue of the metal from the means effecting deformation, such as dies or rolls.

The application of the present invention will hereinafter be illustrated by a number of various examples. Although in these examples it is supposed that the material is worked up on a die extrusion press of the usual construction, it will be well understood that the invention is not limited to said mode of application but is equally applicable to any kind of plastic deformation, particularly to die stamping, rolling, swaging, or drawing operations.

Example 1 On a die extrusion press a round bar of 25 mms. diameter is produced from a block of pure magnesium at a pressing temperature of 450 C. The standard speed, i. e. the speed of the bar issuing from the die when working the press at a standard pace as defined above, amounts to 110 mms. per second. After the finished bar has been allowed to cool the yield strength in tension of the material is found to be 15 kg./sq. mm. and the yield strength in compression 6 kg./sq. mm. whereas the yield strength in torsion amounts to 2.9 kg./sq. mm.

A similar bar is then extruded from another portion of the same material at the same temperature of 450 C. In this case, however, the speed of extrusion is, according to the invention,

' extremely reduced and only amounts to 5 mms.

per second. After normal cooling the yield strength in compression is found to have risen to 11 kg./sq. mm. and the yield strength in torsion to 5.1 kg./sq. mm. whereas the yield strength in tension amounts to 15 kg/sq. mm. and thus remains unaltered when compared with the material extruded in the hitherto usual manner.

Example 2 A magnesium alloy consisting of 6.5 percent of aluminum, 1 percent of zinc, 0.3 percent of manganese, remainder magnesium and minor impurities totalling less than 0.2 percent is extruded at a temperature of 300 C. into round bars of 25 mms. diameter. In accordance with the lower plasticity of this alloy, when compared with pure magnesium, the standard speed of the extrusion is in this case smaller and amounts to about 50 mms. per second. When extruded at this rate the yield strength in tension of the normally cooled material is found to be 22.5 kg./sq. mm., the pressure yield limit is 14 kg./sq. mm. and the fatigue limit (under alternating stresses) is 12 kg./sq. mm.

By extruding another portion of the same block at a lower rate, the speed of the bar issuing from the die being only 10 mms. per second, the conditions of working otherwise being identical, the yield strength in compression is raised to 22 kgL/sq. mm. and the fatigue limit to 16 kg./sq. mm., whereas the yield strength in tension, also in this case, remains unaltered as compared with that of the same material when extruded at the standard rate.

l 10 Example 3 A round bar having 25 mms. diameter and consisting of pure magnesium is extruded at a temperature of 300 C. with a standard pressing speed of 130 mms. per second. The shaped bar when subjected to normal cooling conditions and then tested displays a yield strength in tension of 16 kg./sq. mm., a yield strength in compression of 6.5 kg./sq. mm. and a yield strength in torsion of 3.7 kg./sq. mm.

When, however, according to the alternative mode of carrying out the invention, the bar is actively quenched at its point of issue from the die by means of cold water so as to reduce its temperature throughout to below 200 C., the conditions of working including the speed of extruding being otherwise the same, the material obtained is found to possess a yield strength in compression of 13 kg./sq. mm. and a torsional yield limit of 5.5 kg./sq. mm. whereas the yield strength in tension remains at 16 kg./sq. mm.

Example 4 A block consisting of the magnesium alloy employed in example 2 is extruded into a circular bar of 25 mms. diameter at a temperature of 350 C., the bar being quenched by means of cold water at its point of issue from the die. The speed of the bar issuing from the die amounts to 10 mms. per second. In this case the yield strength in tension is slightly raised and amounts to 24.8 kg./sq. mm., whereas the yield strength in compression is raised to 24.0 kg./sq. mm.

It will be well understood that the effects produced by the two alternative methods as described are practically equivalent. In the case of more massive shapes, however, when it is rather difficult to effect an instantaneous and 15c thorough quenching, it is preferable to produce the desired effect by lowering the speed of deformatlon only. In some cases, however, it is also advantageous to combine both procedures, that is to say, to reduce'the speed of deformation as well as to quench the material at its point of issue from the deforming device.

So far as quenching is concerned it will be well understood that this step is entirely distinct from the quenching step employed in known processes for improving mechanical properties of metals. The application of the known process is limited to the case of alloys which are capable of forming supersaturated solid solutions and the effect of the quenching consists in forcibly maintaining, in the individual crystals, at ordinary temperatures, a state of distribution of the constituents which is not in accordance with the equilibrium prevailing at ordinary temperatures. In contradistinction, the present invention is not limited to alloys forming solid solutions and showing diflerent concentrations of saturation at diil'erent temperatures in these solutions. In order to obtain the efl'ect according to the present invention, however. a plastic deformation is essential and the efiect of the quenching consists in maintaining the size and orientation of the individual crystals, whereas the constitution of the latter is entirely irrelevant.

We claim:

1. A process for improving the yield strength in compression, the yield strength in torsion, and the fatigue limit of technically pure magnesium. which comprises extruding such magnesium at a temperature of about 450 C. and at a speed. measured along the section issuing from the die, of about 5 mms. per second.

2. A process for improving the yield strength in compression, the yield strength in torsion, and the fatigue limit of an alloy consisting of about 6.5 percent of aluminum, about 1 percent of zinc, and traces of manganese, the balance being magnesium, which comprises extruding such alloy at a temperature of about 300 C. and at a speed which is less than one half the maximum speed at which the metal can be extruded at the same temperature and by the application of the same stresses, without cracking.

3. A process for improving the yield strength in compression, the yield strength in torsion, and the fatigue limit of an alloy consisting of about 6.5 percent of aluminum, about 1 percent of zinc, and traces of manganese, the balance being magnesium, which comprises extruding such alloy at a temperature of about 300* C. and at a speed, measured along the section issuing from the die, of about 10 mms. per second.

4. A process for improving the yield strength in compression, the yield strength in torsion and the fatigue limit of magnesium and high percentage magnesium alloys, comprising heating the metal to a selected temperature, placing upon the metal a selected deforming force and plastically deforming said metal at less than one-half of the maximum predetermined speed above which the metal, at said temperature and under said force, will crack.

5. A process for improving the yield strength in compression, the yield strength in torsion and the fatigue limit of magnesium and high percentage magnesium alloys, comprising extruding the metal under a selected deforming force at a temperature between 200 and 500 centlgrade at less than one-half of the maximum predetermined extrusion speed at which the metal, at said temperature and under said force, will not crack.

'6. A process for improving the yield strength in compression, the yield strength in torsion and the fatigue limit of technically pure magnesium,

which comprises extruding the magnesium under 9. selected deforming force at a temperature of about 450 centigrade and at a speed which is less than one-half of the maximum predetermined speed at which the magnesium, at said temperature and under said force, can be extruded without cracking.

WALTHER SCHMIDT. HANS BOTHMANN. JOSEF RUHRMANN.

AlililiiiBLEi cosy the desired effect by lowering the speed of deiormation only. In some cases, however, it is also advantageous to combine both procedures, that is to say, to reduce-the speed of deformation as well as to quench the material at its point of issue (mm the deforming device.

so far as quenching is concerned it will be well understood that this step is entirely distinct from the quenching step employed in known processes for improving mechanical properties oi metals. The application of the known process is limited to the case of alloys which are capable of forming supersaturated solid solutions and the effect of the quenching consists in forcibly maintaining, in the individual crystals, at ordinary temperatures, a state of distribution oi the constituents which is not in accordance with the equilibrium prevailing at ordinary temperatures. In contradistinction, the present inventlon is not limited to alloys forming solid soiutions and showing different concentrations oi saturation at different temperatures in these solutions. In order to obtain the eflfect accord ing to the present invention, however, a plastic deformation is essential and the effect of the quenching consists in maintaining the size and orientation of the individual crystals, whereas the constitution of the latter is entirely irrelevant.

We claim:

1. A process ior improving the yield strength the yield strength in torsion,

slum. which slum at a temperature of about 450 C. and at a speed. measured along the section issuing from the die, of about 5 mms. per second.

2. A process for improving the yield strength in compression, the yield strength in torsion, and the fatigue limit of an alloy consisting 01 about 6.5 percent of aluminum, about 1 percent oi zinc, and traces of manganese, the balance being magnesium, which comprises extruding such alloy at a temperature of about 300 C. and at a speed which is less than one halt the maximum speed at which the metal can be extruded at the same temperature and by the application of the same stresses, without cracking.

3. A process for improving the yield strength in compression, the yield strength in torsion, and the fatigue limit of an alloy consisting of about 6.5 percent of aluminum, about 1 percent of zinc, and traces of manganese, the balance being magnesium, which comprises extruding such alloy at a temperature of about 300 C. and at a speed, measured along the section issuing from the die, of about 10 mms. per second.

4. A process for improving the yield strength in compression, the yield strength in torsion and the fatigue limit oi. magnesium and high percentage magnesium alloys, comprising heating the metal to a selected temperature, placing upon the metal a selected deforming force and plastic-ally deforming said metal at less than onehalf of the maximum predetermined speed above which the metal, at said temperature and under said force, will crack.

5. A process for improving the yield strength in compression, the yield strength in torsion and the fatigue limit of magnesium and high percentage magnesium alloys, comprising extruding the metal under a selected deforming force at a temperature between 200 and 500 centigrade at less than one-half of the maximum predetermined extrusion speed at which the metal, at said temperature and under said force, will not crack.

6. A process for improving the yield strength in compression, the yield strength in torsion and the fatigue limit of technically pure magnesium, which comprises extruding the magnesium under a selected deforming force at a temperature of about i50 centigrade and at a speed which is less than one-half of the maximum predetermined speed at which the magnesium, at said temperature and under said force, can be extruded without cracking.

WALTHER. SCHMIDT. HANS BOTHMANN. JOSEF RUHRMANN.

trlinrlclih: or ooiiktchou.

Patent No. l, 923, 591.

August 22, I933.

WALTHER SCHMIDT. ET AL.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction 131, for "torsional yield limit" read case in the Patent Oliice. Signed and sealed this (Seal) "yield for "circular" read "round": and that the said Letters with these corrections-there in that the some may as follows: Page 2, lines 130 and strength in torsion"; and line 136,

Patent should be read conform to the record of the 24th day of October, A. D. 1933.

F. M. Hopkins Acting Conmissioner of Patents.

CERTIFICATE or CORRECTION.

Patent No. 1,923.59]. August 22. 1933.

WALTHER SCHMIDT. ET AL.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2, lines 130 and 131. for "torsional yield limit" read "yield strength in torsion"; and line 136, for "circular" read "round"; and that the s..id Letters Patent should be read with these corrections there in that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 24th day of October, A. D. 1933.

F. M. Hopkins (Sell) Acting Cormniasioner of Patents. 

